To perform rectification and smoothing to obtain a direct current from an AC power supply, a configuration using a diode bridge and a smoothing capacitor is simplest. Meanwhile, since this configuration results in a rectifier circuit of so-called capacitor input type in which an input current flows only when whose power supply voltage is near the peak, which causes a lowering of power factor or an increase of input harmonic. Since the matter of input harmonic current is regulated by international standards, countermeasures according to the input power have been required.
With respect to this trend, as a switching power supply, various converters including one called power factor correction (PFC) converter or high power factor converter having a corrected power factor have been proposed.
Recently, in the field of electric home appliance or information equipment, a trend requiring cost reduction of this PFC converter has been pronounced, and an effort for reduction in the number of pins of a PFC control IC or reduction in the number of parts of the converter has been made.
As an example of this trend, there are Japanese Patent Application Laid-Open Publication No. 2006-510340 (Patent Document 1 of International Rectifier Corporation) and a control IC of International Rectifier Corporation IR1150 (IR1150 Data Sheet (2005) and the Power Factor Correction control IC of AC-DC converter: μPFC IC “IR1150” series of International Rectifier Corporation (Non-Patent Documents 1 and 2)). These proposals are intended to omit outside parts and achieve low loss and generally do not require sensing (or detecting) an input voltage waveform, which is required for performing the PFC control, into the control IC.
Here, with reference to FIG. 14, a circuit configuration of a conventional switching power supply will be explained. FIG. 14 is a circuit diagram showing a configuration of a conventional switching power supply, in which IR1150 which is a control IC of International Rectifier Corporation is used. Hereinafter, the configuration and operation of the circuit will be explained.
In FIG. 14, an AC power supply 1 gives an input voltage 4 via an input filter 2 and a rectifier 3 to have a full wave rectified waveform. Since the capacitance of a capacitor 12 is relatively small, a sinusoidal waveform is hardly smoothed. A series body of a coil 5, a power MOSFET 8, and a current-detecting resistor 16 is connected on the direct current side of the rectifier 3.
A series body of a boost diode 6 and a smoothing capacitor 9 is connected between the drain and source of the power MOSFET 8. Not only voltage-dividing resistors 11e, 11f but a load 10 is connected on both sides of the smoothing capacitor 9. The voltage of the smoothing capacitor 9 is called output voltage 7. An output-voltage-sense signal 14 is outputted from the middle point of the voltage-dividing resistors 11e and 11f, and inputted to a control circuit 20 mounted as a control IC.
Inside the control circuit 20, the output-voltage-sense signal 14 is subtracted from an output-voltage-command value Vref, and the error obtained by the subtraction is inputted to an amplifier 23 to be subjected to error amplification. The output from the amplifier 23 is inputted to a reset integrator 29, and an output from the current-detecting-resistor 16 is subtracted from the amplified output, and the output after subtraction is inputted to the − (negative) terminal of a comparator 25.
The output of the reset integrator 29 is inputted to the + (positive) terminal of the comparator 25 to be compared with the input value of the − terminal. The output of the comparator 25 is inputted to the reset of a flip-flop circuit 27. A clock 26 is inputted to the set of the flip-flop circuit 27. The Q output of the flip-flop circuit 27 is connected to a driver 17 outside the control circuit 20.
The output of the driver 17 is connected to the gate of the power MOSFET 8. The Q bar output of the flip-flop circuit 27 is connected to the reset switch of the reset integrator 29.
In this circuit configuration, according to changes of a voltage feedback value and a current detecting value, control of changing a pulse width for each cycle is adopted. As a result, even if sensor of an input voltage waveform is omitted, the current of the coil 5 has a substantially sinusoidal waveform synchronizing with an input voltage.